The human ear may be divided into the outer ear, middle ear, and inner ear. The middle ear includes the eardrum and the auditory ossicles—hammer, anvil, and stirrup. Sound waves entering the outer ear cause the eardrum to oscillate. A mechanical impedance conversion occurs in the middle ear, which allows an optimum transmission of the sound signal from the outer ear to the inner ear. Thus, the ear drum oscillations are transmitted by the ossicles to the oval window of the inner ear which vibrates the fluid within the cochlea. Hair cells projecting into the cochlea are bent by the vibration of the cochlear fluid, thereby triggering nerve pulses.
The middle ear also contains the tympanic muscle and the stapedius muscle. The tympanic muscle is linked to the hammer, the stapedius muscle being connected via a tendon to the stirrup. In case of an excessively high sound pressure which could damage the inner ear, both muscles contract reflexively to decrease the mechanical coupling of the eardrum to the inner ear (and thereby also the force transmission). This protects the inner ear from excessively high sound pressures. This tensing of the stapedius muscle when triggered by high sound pressures is also referred to as the stapedius reflex. Medically relevant information about the functional capability of the ear may be obtained from the diagnosis of the stapedius reflex. The measurement of the stapedius reflex also is useful for setting and/or calibrating cochlear implants, because the sound energy perceived by a patient may be deduced from the measured stapedius reflex.
The stapedius reflex can be determined in a post-operative clinical setting using an acoustic tympanometer which also requires another additional device to take and use the electrical measurements. To measure the stapedius reflex, it is known to intra-operatively use electrodes that are brought into contact with the stapedius muscle to relay to a measuring device the action current and/or action potentials generated upon a contraction of the stapedius muscle. A reliable minimally-invasive contact of the stapedius muscle is difficult because the stapedius muscle is situated inside a trough present in a bone and only the tendon of the stapedius muscle connected to the stirrup and its upper part are accessible from the interior of the middle ear.
Various intraoperative stapedius muscle electrodes are known from U.S. Pat. No. 6,208,882, however, these only achieve inadequate contact of the stapedius muscle tissue (in particular upon muscle contraction) and are also very traumatizing. In order to make ESRT measurements simpler and quicker, first non-commercial intraoperative experiments and studies have been conducted with monopolar (Almqvist et al. 2000) or bipolar hook electrodes (Pau et al. 2008), respectively, which have been attached at the stapedius tendon or muscle to measure the muscle activity in the case of a reflex. The measurements were successful, but the electrode design was only suitable for intra-operative tests. The Almqvist hook electrode does not allow a quick and easy placement at the stapedius tendon and muscle—the electrode has to be hand held during intra-operative measurements. the Pau bipolar hook electrode does not ensure that that both electrodes are inserted into the stapedius muscle due to the small dimensions of the muscle and the flexibility of the electrode tips. One weakness of these electrodes is that they do not qualify for chronic implantation.
DE 10 2007 026 645 A1 discloses a two-part bipolar electrode configuration where a first electrode is pushed onto the tendon of the stapedius muscle or onto the stapedius muscle itself, and a second electrode is pierced through the first electrode into the stapedius muscle. One disadvantage of the described solution is its rather complicated handling in the very limited space of a surgical operation area, especially manipulation of the fixation electrode. In addition, the piercing depth of the second electrode is not controlled so that trauma can also occur with this approach.
U.S. patent application Ser. No. 12/763,374, filed Apr. 20, 2010 (incorporated herein by reference) describes an electrode arrangement 100 as shown in FIG. 1 for sensing electrical activity in target tissue. A support electrode 101 has an elongate electrode body with a base end 102 and a penetrating end 103 for insertion into the target tissue. A fixation electrode 104 has an elongate electrode body with a base end 105 and a penetrating end 106 at an angle to the electrode body. The penetrating electrode 104 passes perpendicularly through an electrode opening 107 in the support electrode 101 so that the penetrating ends 103 and 106 of the electrodes penetrate into the target tissue so that at least one of the electrodes senses electrical activity in the target tissue.
It would be advantageous to have a simple cost effective electrode for measuring action currents and/or action potentials in electrically active tissues (such as the stapedius muscle tissue), which enables secure but reversible fixing of the electrode in the target tissue, but which traumatizes the tissue as little as possible.